Conventionally, an aromatic polyamide (aramid) fiber comprising aromatic dicarboxylic acid components and aromatic diamine components, particularly p-aramid fiber, is widely used in industrial applications and clothing material applications by taking advantage of characteristics such as its strength, high modulus and high heat resistance.
For example, a protective clothing material using a multi-layered of aramid fiber cloths is one of the specific applications, and various methods are hitherto proposed in, for example, JP-A-52-46700 and JP-A-57-207799.
A poly(p-phenylene terephthalamide) (PPTA) fiber is the representative p-aramid fiber used in the above applications. This fiber has many advantages, but has the disadvantage on process in the point that the fiber is produced by a so-called crystal liquid spinning method utilizing optical anisotropy of a polymer dope. Furthermore, regarding fiber performances, the fiber has the disadvantages such that among mechanical properties, strength is not always high, elongation is low and toughness is deficient.
Particularly, interest regarding protection against bullet is recently increased, and the requirement in a protective clothing material to firearms firing a bullet having energy of higher level is increasing. To meet this requirement, PPTA fiber is required to increase the number of layers in order to increase bulletproof performance, and this significantly increases the weight. As a result, it is difficult to regularly use the PPTA fiber as a protective clothing material.
To eliminate such problems, an attempt has been made to develop an aromatic polyamide that has high solubility in the conventional amide solvent and can easily be spun, and that a fiber obtained therefrom has high tensile strength and high initial modulus after drawing treatment.
For example, JP-A-7-300534 proposes production of an aromatic polyamide fiber which forms an isotropic solution to an amide solvent by polycondensing dicarbonyl dihalides and at least two kinds of diamines.
However, the aromatic polyamide fiber disclosed in the above patent publication is deficient in mechanical properties such as tensile strength and initial modulus, and it is the actual condition that an aramid fiber that can develop excellent mechanical properties is not yet obtained.